In order to control brain-machine interface according to intention of a person by using an EEG signal, parameters for cognitive properties of brainwaves are analyzed and used.
When an EEG signal is used as an interface control signal as described above, the EEG signal can be analyzed by an analytic method based on a time or frequency axis. In this case, the analysis method based on the time axis repeatedly measures an EEG signal associated with stimulus presentation, to arrange unit EEG segments relative to the time point of the stimulus onset, and then, calculate an average EEG potentials based on the stimulus-onset time point. This method uses the principle that only the EEG signal associated with the stimulus (or event) is effective for the average value and survives after averaging, and an EEG signal irrelevant to the stimulation is canceled out by the average.
A brain potential, which is obtained by being accumulated from an average value in relation to a presentation of a stimulus or an event, is called an ‘event-related potential (ERP).’ With respect to a representative EEG component obtained by the EEG analysis based on the time axis, there is a ‘steady state visual evoked potential (SSVEP).’ The SSVEP component is an EEG signal using EEG responding to repeatedly flickering visual stimuli. For example, when a person watches a flickering stimulus, a spectral EEG component at the same frequency as the flickering-stimulus frequency is physically driven. In this case, the oscillatory EEG activity, which is induced by the flickering visual stimulus and has the same frequency as that of the stimulus, corresponds to SSVEP.
In this regard, Korean Patent Application Publication No. 2013-0002590 (Title of Invention: QWERTY-Type Text Input Interface Device Using Steady State Visual Evoked Potential and Text Input Method) describes a text input interface device, which includes a text display that displays a multiple number of texts in a QWERTY style, an EEG signal measuring unit that measures an EEG signal of a user during the time when a steady state visual evoked potential is evoked by a visual flickering stimulus resulting from the displayed texts, an EEG analyzing unit that analyzes the measured EEG signal, and a text outputting unit that outputs texts corresponding to the analyzed EEG signal.
However, the above conventional text input interface device inevitably induces eye-movement depending on where a user puts his/her attention. In other words, this technique can be conveniently replaced by an eye-tracker device, which can yield further higher accuracy than EEG signals. This is because a user of this technique must overtly move his/her eye focus onto a corresponding keystroke of a keyboard, so as to acquire EEG components for the corresponding letter. Accordingly, the conventional text input interface device has such limitations since it cannot provide correct information (i.e., a user's intention) if a user covertly imagines a certain type of letters without moving his/her eyes.